EP0893827B1 - Electronic device and method for forming a membrane for an electronic device - Google Patents

Description

Field of the Invention

This invention relates in general to electronic devices and a method forforming electronic devices. More particularly, this invention relates to lowpower electronic devices and a method for their manufacture.

Background of the Invention

A chemical sensor is a device which monitors the concentration of agiven chemical species in a liquid or a gas. Chemical sensor devices comprise asensitive layer, which is sensitive to a particular chemical which is to bedetected by the sensor device, and a heater integrated on a semiconductorsubstrate such as a silicon substrate. The heater increases the temperatureof the sensitive layer to increase the sensitivity of the sensor device and istypically required to heat the sensitive layer to temperatures in the range of25ºC to 600ºC during operation of the sensor device. At these temperatures,there is significant loss of thermal energy through the silicon substrate andtherefore such devices suffer from high power consumption.

The high power consumption is a particular problem when the sensordevice is required to be powered by a battery. For example, some applicationsmay require battery back-up operation. For such battery poweredapplications, the power consumption should be around 60mw at 400ºC in DCmode.

In order to reduce the power consumption in semiconductor chemicalsensors, it has been proposed, see for example French Patent Application no.FR-A-2615287, to micromachine the backside of the bulk silicon substrate toform a thin membrane under the active region of the sensor device (i.e. underthe heater and sensitive layer). The thin membrane is formed by depositing asolution of Boron Oxide (B₂O₃) by spin-on onto the silicon substrate and by thendiffusing the boron dopant into the substrate. Although this technique reducesthe thermal losses to the bulk silicon substrate, since the membranecomprises a silicon layer doped with P+ type material, which material has a high thermal conductivity (35Wm^-1K^-1), the thermal losses during use of thesensor, and hence power consumption, are still too high for low power operation,such as battery back-up operation. For example, at 400ºC, the powerconsumption for such a sensor can be as high as 200mW in DC mode.

Another technique for reducing thermal loss is described in US PatentNo. 5,545,300. This patent describes using a composite membrane comprisinga glass film formed over a silicon nitride film. The reduction in the powerconsumption with the described technique is limited due to the high thermalconductivity of the silicon nitride film (23Wm^-1K^-1) used in the compositemembrane. The power consumption of the device disclosed in this patent isclose to 110mW at 400ºC in DC mode, which is still too high for batteryoperation.

Since two different steps (deposition of silicon nitride film followed bydeposition of silicon oxide film) are required to realise the membrane, thistechnique also suffers from the cost disadvantage mentioned above of havingseveral fabrication steps. A further disadvantage of the process described inthis patent is that as the silicon nitride layer is deposited on top of the siliconsubstrate, such a process step can create dislocations on the silicon surfacewhich is not compatible with integrated circuit (IC) technology. In other words,with such a process, it would not be possible to integrate the control chip on thesame substrate as the sensor device.

An article entitled 'Reduction of heat loss of silicon membranes by theuse of trench-etching techniques' by J. Werno, R. Kersjes, W. Mokwa and H.Vogt, and published on pages 578-581 of Sensors and Actuators A, 41-42(1994), describes a solution to improve the thermal insulation of single crystalsilicon membranes by means of oxide-filled trenches. The power consumptionof this technique is 230mW at 400°C in DC mode, which is too high for batteryoperation. In addition, the described trench-etching technique employs threesuccessive process steps (SIMOX/epitaxy/oxide-filled trenches) and is thus anexpensive technique to use with respect to the manufacturing costs.

There is therefore a need to provide an improved electronic device whichcan be operated at low power and a method of forming such an electronicdevice.

Summary of the Invention

In accordance with a first aspect of the present invention there isprovided an electronic device as recited in claim 1 of the accompanying claims.

In accordance with a second aspect of the present invention there isprovided a method of forming an electronic device as recited in claim 5 of theaccompanying claims.

Brief Description of the Drawings

An electronic device and a method for forming an electronic device inaccordance with the present invention will now be described, by way of exampleonly, with reference to the accompanying drawings in which:

FIGs. 1-9 show simplified schematic cross-sectional diagrams of aportion of an electronic device in accordance with the present invention duringvarious stages of fabrication; and

FIG. 10 shows a graph of power consumption as a function oftemperature for different types of membranes.

Detailed Description of the Preferred Embodiment

In the following description, the invention will be described in relation to achemical sensor device. It will however be appreciated that the presentinvention may apply to any other electronic devices, such as thermal sensors,calorimetric sensors, pressure sensors, microphones, inkjet sensors,micropumps, and microsystems, which use a membrane to support the activeregion of the electronic device over a cavity in the bulk substrate.

A method for forming a chemical sensor device in accordance with apreferred embodiment of the present invention will now be described withreference to FIGs. 1-9 of the drawings which are simplified schematic cross-sectionaldiagrams of a portion of the chemical sensor device during variousstages of fabrication. Although in the following description the semiconductorsubstrate, layers and regions will be described as having certain conductivitytypes and being comprised of certain material, this is for illustrative purposes only. It is not intended that the invention be limited to the specific conductivitytypes nor the specific materials referred to herein.

Referring firstly to FIG. 1, asemiconductor substrate 2, preferably asilicon <100> substrate, is provided. Thesemiconductor substrate 2 has afirstsurface 4 and asecond surface 6, which is opposite thefirst surface 4. Adielectric layer 8 of an oxy-nitride material is formed oversemiconductorsubstrate 2 on thefirst surface 4, see FIG. 2. Thedielectric layer 8 has athickness 10 in therange 30 nanometres to 3 micrometers.

In the preferred embodiment, thedielectric layer 8 is formed of a siliconoxy-nitride material having a composition SiO_xN_y. The composition (i.e. thevalues of x and y) is selected so that thedielectric layer 8 has good mechanicalproperties and low thermal conductivity in the range 1-30 Wm^-1K^-1. (preferably5 Wm^-1K^-1). Adielectric layer 8 having good mechanical properties ensures highmanufacturing yield of the device (up to 100%).

The mechanical properties of thedielectric layer 8 depend upon theYoung's Modulus of the silicon oxy-nitride material, which forms thedielectriclayer 8, and the thermal expansion of the silicon oxy-nitride material. Thecomposition of the silicon oxy-nitride material is selected so that the thermalexpansion of the material is substantially equal to that of the substratematerial. In the preferred embodiment which has a silicon substrate, thecomposition is chosen so that the thermal expansion of the silicon oxy-nitridematerial is substantially equal to the thermal expansion of silicon (that is, 2.5X 10^-6/ºC). This ensures that thedielectric layer 8 has a low residual stresslevel.

The composition of the silicon oxy-nitride material is also selected sothat the Young's Modulus is close to that of the substrate material. Sincesilicon has a Young's Modulus of 170 GPa, the silicon oxy-nitride material isarranged to have a Young's Modulus in the range 100-180 GPa.

Thedielectric layer 8 can be formed by Plasma Enhanced ChemicalVapour Deposition (PECVD) or Chemical Vapour Deposition (CVD). PECVDis preferred because it is easier to control the composition of the oxy-nitridematerial by adjusting the pressure and power of the plasma and hence the oxy-nitridedielectric layer 8 can be optimised both for low thermal conductivity andfor low stress.

In the preferred embodiment, a mixture of silane, ammonia, nitrous oxideand nitrogen in a 0.17:0.14:0.14.0.55 ratio respectively is used in a PECVDprocess to provide adielectric layer 8 having a composition SiO_xN_y, where x =0.89, and y = 0.74. The PECVD process is carried out in a CVD 5000 chambersupplied by Applied Materials Inc., which chamber is heated to a temperatureof 400ºC. The plasma pressure is 600 Pa (4.5 Torr) and the plasma power is 325 W.

Referring now also to FIG. 3, asilicon oxide layer 12 is formed on thesecond surface 6 of thesemiconductor substrate 2. Thesilicon oxide layer 12can be formed by thermal oxidation of the silicon substrate or by PECVD or byCVD. Thermal oxidation is preferred because during the oxidation of the siliconsubstrate, thedielectric layer 8 is annealed at the same time.

A conductive layer is formed over thedielectric layer 8. The conductivelayer is then patterned and etched to leave aportion 14 of the conductive layerwhich forms aheater 14 of the chemical sensor device, see FIG. 4. Theconductive layer comprises a polysilicon layer or a metal layer having goodconductivity: the resistivity is in therange 10^-6 ohm.cm to 10^-3 ohm.cm. Thethickness 16 of theconductive layer 14 is between 30 nanometres and 3micrometres.

In FIG. 5, amasking layer 18 is formed over thesilicon oxide layer 12 onthe backside of thesemiconductor substrate 2. Thethickness 20 of themasking layer 18 is between 30 nanometres and 3 micrometres. In thepreferred embodiment, a silicon nitride masking layer is PECVD deposited onthesilicon oxide layer 12 using the same equipment used for PECVD depositionof thedielectric layer 8. Thesilicon oxide layer 12 is required since the siliconnitride masking layer 18 has poor adherence to thesilicon substrate 2. Themasking layer 18 is used as a backside bard mask during backsidemicromachining of thebulk silicon substrate 2. Alternatively, a silicon oxy-nitridelayer could be used as themasking layer 18 due to the very low etchrate of silicon oxy-nitride in potassium hydroxide solution, which solution is usedto etch thesemiconductor substrate 2.

A first insulatinglayer 22 is then deposited over thedielectric layer 8and theportion 14 of the conductive layer. In the preferred embodiment, thefirst insulatinglayer 22 comprises a silicon dioxide layer, which is deposited onthedielectric layer 8 by PECVD or a Low Pressure Chemical Vapour Deposition LPCVD. The first insulatinglayer 22 is patterned and etched toformcontact openings 24 extending to the conductive layer (FIG. 6).

In FIG. 7, themasking layer 18 is patterned and then themasking layer18 and thesilicon oxide layer 12 are etched to form anopening 26 extending tothesecond surface 6 of thesemiconductor substrate 2. Theopening 26 definesthe area for etching thesemiconductor substrate 2.

Asensitive layer 30 is then formed on a portion of the first insulatinglayer 22 so that it overlies the heater 14 (FIG. 8). Thesensitive layer 30 isformed by first depositing (by sputtering, evaporating, CVD or spin-on) a layerof material which is sensitive to the chemical species to be sensed by thesensor device over the whole of the device and then using patterning to removeportions of the layer to leave thesensitive layer 30. In the preferredembodiment, the sensitive material is a metal oxide and a standard lift-offtechnique is used. Polymeric sensing materials may also be used.

Electrical contact is then made to theheater 14 and thesensitive layer30. Metallisation, such as chromium/titanium/platinum metallisation, isformed over the wafer by evaporation. The metallisation is then patterned bylift-off or etching to leave heater metal conductors 28 (FIG. 8) and sensitivelayer metal conductors (not shown).

In order to improve thermal isolation of the active region of the chemicalsensor device, which region includes thesensitive layer 30 and theheater 14,thesemiconductor substrate 2 is etched or bulk-micromachined through theopening 26 to form acavity 32 which extends from thesecond surface 6 to thefirst surface 4 of thesemiconductor substrate 2 as shown in FIG. 9. Thesingledielectric layer 8 of oxy-nitride material forms the membrane of the chemicalsensor device in accordance with the present invention, which membraneextends at least across thecavity 32 and supports the active region of thechemical sensor device.

The bulk-micromachining process is an anisotropic etch process. In thepreferred embodiment, a wet etch solution comprising potassium hydroxide(KOH) is used to remove the bulk of thesilicon substrate 2. Since oxy-nitridematerial has a very low etch rate in KOH (0.04 micrometres per hour), thedielectric layer 8 acts as an etch stop layer. Table 1 below gives data on theKOH mechanical yield and etch rate at 90ºC for different kinds of membranes.

Membrane Material	KOH Yield (%)	KOH Etch Rate (µm/h)
Oxide	20	0.25
Oxy-Nitride	100	0.04
Nitride/Oxide	95	0.02
Trenches	N/A	0.25
P+ Silicon	100	15

This very low etch rate means that the thickness of thedielectricmembrane 8 can be accurately controlled, and hence the same powerconsumption of the sensor device can be controlled, over a wafer, wafer towafer and lot to lot. As can be seen from the etch rates given in Table 1, this isnot the case with a P+ silicon layer which forms the membrane in the abovedescribed French Patent Application No. FR-A-2615287.

Table 1 also shows the mechanical yield for membranes of differenttypes of material. An oxy-nitride membrane deposited by PECVD inaccordance with the present invention, as can be seen from Table 1, has goodmechanical properties.

In the preferred embodiment described above, a portion of the siliconsubstrate is removed by backside micromachining using a wet etch process.Other techniques for removing the silicon substrate so as to leave a membraneover a cavity may also be used. For example, the silicon substrate may besurface micromachined using a wet etch process (on first surface 4), or thesilicon substrate may be backside or surface micromachined using a dry etchprocess, such as plasma etching, or a direct wafer bonding technique may beused. For the later technique, two wafers are bonded together and then onewafer is micromachined to provide the membrane.

The oxy-nitride membrane 8 of the present invention provides low heatconduction after bulk substrate removal and also acts as an insulating layerbetween the heater and the substrate. The effectiveness of the thermal isolation provided by the membrane is determined by the thermal conductivityof the PECVD oxy-nitride membrane, which mainly depends on thecomposition of the membrane.

FIG. 10 shows a graph of the power consumption as a function oftemperature for different types of membrane. As shown bycurve 40, the oxy-nitridemembrane in accordance with the present invention meets the lowpower requirements for battery operation across a wide temperature range.

In summary, the present invention provides a device having amembrane to support the active region of the device comprising a singledielectric layer of oxy-nitride material having low thermal conductivity (i.e. lowpower consumption), low stress level, very low etch rate in KOH and which onlyrequires a single process step. The invention therefore provides a membranewhich can support low power operation, such as battery back-up, and which issimple and not expensive to manufacture.

Claims (11)

1. An electronic device comprising:

   a semiconductor substrate (2) formed of a semiconductor material andhaving a cavity (32) extending into the substrate;

   a membrane (8) formed over the semiconductor substrate so as to extendacross the cavity in the semiconductor substrate; and

   an active region (14, 22, 30) supported by the membrane (8) andpositioned adjacent the cavity (32),

   the electronic device beingcharacterized in that the membrane (8)comprises a single dielectric layer (8) formed of a silicon oxy-nitride materialhaving a composition SiO_xN_y, wherein x, and y are selected such that the siliconoxy-nitride material has a Young's Modulus substantially the same as theYoung's Modulus of the substrate semiconductor material, a thermal expansionsubstantially the same as the substrate semiconductor material and a thermalconductivity in the range 1-30 Wm^-1K^-1.
2. An electronic device according to claim 1 wherein the oxy-nitride materialhas a composition SiO_xN_y, where x = 0.89, and y = 0.74.
3. An electronic device according to any preceding claim wherein theelectronic device comprises a semiconductor sensor device and the active regioncomprises:

   a conductive layer (14) formed over the membrane;

   an insulating layer (22) formed over the conductive layer; and

   a sensitive layer (30) formed over the insulating layer.
4. An electronic device according to any preceding claim wherein thesemiconductor substrate has a first surface (4) and a second surface (6) andwherein the cavity (32) extends through the substrate between the secondsurface (6) and the first surface (4).
5. A method for forming a membrane for supporting an active region (14, 22,30) of an electronic device, the method comprising the steps of:

   providing a semiconductor substrate (2) formed of a semiconductormaterial;

   forming a membrane (8) over the semiconductor substrate;

   forming the active region of the electronic device over the single dielectriclayer (8); and

   removing a portion of the semiconductor substrate so as to provide acavity (32) in the substrate, the single dielectric layer (8) extending across thecavity (32), the method beingcharacterized in thatthe membrane (8) comprises asingle dielectric layer of a silicon oxy-nitride material having a compositionSiO_xN_y, wherein x, and y are selected such that the silicon oxy-nitride materialhas a Young's Modulus substantially the same as the Young's Modulus of thesubstrate semiconductor material, a thermal expansion substantially the same asthe substrate semiconductor material and a thermal conductivity in the range 1-30Wm^-1K^-1.
6. A method according to claim 5 wherein the semiconductor substrate has afirst surface (4) and a second surface (6), wherein the step of forming themembrane (8) comprises forming the membrane over the first surface of thesemiconductor substrate and wherein the step of removing a portion of thesemiconductor substrate comprises the steps of:

   forming a masking layer (18) over the second surface (6) of thesemiconductor substrate after forming the membrane;

   removing a portion of the masking layer so as to provide an opening (26)extending through the masking layer to the second surface; and

   etching the semiconductor substrate through the opening so as to providethe cavity (32) extending between the second surface and the membrane.
7. A method according to claim 6 wherein the etching step comprises wetetching the semiconductor substrate with a solution comprising potassiumhydroxide.
8. A method according to claim 6 or 7 wherein the semiconductor substratecomprises a silicon substrate and the masking layer comprises a silicon oxidelayer over the second surface and a silicon nitride layer over the silicon oxidelayer.
9. A method according to claim 5, 6, 7, or 8 wherein the forming a membranestep comprises depositing a single dielectric layer of a silicon oxy-nitride materialson the semiconductor substrate.
10. A method according to claim 5, 6, 7, 8, or 9 wherein the oxy-nitridematerial has a composition SiO_xN_y, where x = 0.89, and y = 0.74.
11. A method according to claim 5, 6, 7, 8, 9 or 10 wherein the electronicdevice is a semiconductor sensor device, and the method further comprises thesteps of:

    forming a conductive layer (14) on a portion of the single dielectric layer(8);

    forming a first insulating layer (22) over the single dielectric layer (8) andthe conductive layer (14);

    removing portions of the first insulating layer (22) to form contact openings(24) extending to the conductive layer (14);

    forming a sensitive layer (30) over a portion of the first insulating layer (22)such that the sensitive layer extends over the conductive layer but not over thecontact openings (24); and

    forming electrical contacts to the conductive layer and the sensitive layer.

Images